The use of compact disk (CD) data storage is well known in the art. CDs comprise a data layer residing beneath a clear coating layer having a well defined thickness. The data layer has marks of varying reflectance which are read by a laser beam focused on the data layer. The laser beam must be focused to a spot of minimal size in order for the system to achieve maximum data density, which is desired.
In order for the reading laser beam to be focused to a spot of minimal size, the optical system which focuses the laser beam must be designed to compensate for the distorting effects of spherical aberration. Spherical aberration has the effect of enlarging the focused spot size, which precludes achieving maximum data density.
The amount of spherical aberration varies with the thickness of the clear coating layer. For CDs having a single data layer and a clear coating of a known thickness, a lens assembly can be designed to compensate for the well defined magnitude of spherical aberration present. An optical focusing system designed for a first disk having a given clear coating thickness, however, cannot necessarily be used on a second disk having a different clear coating thickness.
It is expected that future optical data storage systems may use optical disks that have multiple data layers located at different depths. Such a system will require an adjustable spherical aberration compensation capability. The amount of spherical aberration compensation provided will need to be different for each data layer. More generally, spherical aberration requires correction in any system which must focus light to a minimal spot size at various depths within a medium. Such systems are not limited to current optical data storage systems.
U.S. Pat. No. 5,202,875 to Rosen et al. discloses an optical data storage system having multiple data layers. Different embodiments of the data storage system employ different methods for providing spherical aberration correction. In one embodiment, stepped plates are disposed between an objective lens and a CD. The stepped plates have different thicknesses and are moved in and out of the optical path such that light always passes through the same thickness of material (clear coating material) before hitting the data layer. Paired wedges and rotatable stepped wedges also perform the same function.
In another embodiment, Rosen describes an aberration compensation technique involving the use of two lenses (a convex lens and a concave lens) in addition to the objective lens assembly. The lenses are moved relative to one another to provide controlled spherical aberration correction. The distance between the lenses determines the magnitude of aberration correction. An inherent requirement of this technique is that the spherical aberration compensation lenses must be used in conjunction with an objective lens head which provides focusing. U.S. Pat. No. 5,610,901 to Best et al. discloses a similar spherical aberration compensation technique.
U.S. Pat. No. 5,157,555 to Reno discloses a spherical aberration correction apparatus which uses two lenses having an adjustable air gap between them. The lenses are convex and concave, with complementary surfaces facing each other. Reno's apparatus is used in conjunction with an optical head for focusing a beam onto the data surface. The focusing and spherical aberration compensation lenses are separate components. This increases the number of optical elements required in the total system, thus increasing system complexity.
It would be a significant advance in the art of optical data storage to provide an aberration correction system which requires fewer optical components while providing adjustable spherical aberration correction.